What is the minimum size requirement for a Fluid mesh object in a Fluid simulation?
This is dependent on resolution. 

Imagine the domain as a grid of 3D pixels (volumetric pixels, or voxels). If your fluid object is smaller than a voxel, then it can't be represented properly (even if it's two voxels, that's still not good)



To fix this, you can either increase the resolution (this will increase bake time), or scale down the domain (this shouldn't increase bake time, but obviously there is less room for the fluid).  

Since the resolution defines a fixed number of voxels, scaling it down will give you the same number of voxels, but they will be smaller (making the fluid object relatively larger):





There may be a hard limit of how small you can go eventually, but anything remotely reasonable should be fine.